Tropical fruits such as papaya have been cultivated commercially for decades in tropical countries. In recent times but before the increased efficiency and modernization of communications and world trade, people and products in general moved from one country to another in a matter of hours. Fruits in particular, as commercial products, today move more rapidly and are available to consumers in countries far distant from their place of production. This benefit brings with it an inherent problem in that plant diseases are also transported very easily. Long ago the diseases of one region were virtually unknown elsewhere, but today this is not the case, with some diseases even spreading throughout the globe.
Recently, viral infections have become the main problem in cultivated plants, especially vegetables and fruits. One of the most striking examples is that of the papaya, since the Papaya Ringspot Virus (PRV), first identified in 1945 and at first called Watermelon Virus No. 1, has become a significant problem for papaya plantations worldwide, and has brought the industry nearly to collapse in various locations, including Hawaii in 1994. This virus, which is transmitted from sick plants to healthy ones by sap sucking insects, such as the common aphid (aphis spp. and mizus spp. and others), is a key concern for various reasons, the first being its lethal and destructive capacity and the speed of contagion. The second reason is that at the present time there is no cure for viral diseases in plants, and the third is that besides the papaya, this virus attacks other cultivated plants such as the cucurbitaceous family that are of great economic importance, such as watermelon, squashes and peppers. It is very dangerous to cultivate these plants near papaya plantations infected with PRV because contagion can be immediate, resulting in greatly reduced and poor quality harvests.
The inverse is also true: new papaya plantations cannot be installed near old fields of cucurbitaceous plants where PRV is present. Another problem is that the virus also attacks common weeds that may go unnoticed but that act as carriers of the virus from which it passes to cultivated plants. Yet another problem is that lack of knowledge about the disease and its function among the great majority of farmers in the world and the small number of specialists knowledgeable about the disease and the difficulty in obtaining their assistance. The result is that now practically all the tropical regions of the world are fighting this problem.
Two decades ago papaya growers resolved the PRV problem by planting their papayas in places that were free of the problem, but with the reduction of such places they have resorted to the traditional practice of applying various insecticides to kill the insect vectors of the illness. However, the speed of movement and reproduction of these insects nullifies the effectiveness of the pesticides. In actual experiments, not even the application of aphid-specific insecticides every other day for a period of 4 months was able to control the disease. This practice has thus been disproved yet no other solution has arisen. In spite of this, papaya growers continue to rely on the use of pesticides, in spite of uncertain results, and with the potential for damaging the ecosystem. Most recently in Mexico, both older antibiotics and new products of doubtful origin have been advertised to offer a cure or at least the reversal of symptoms in the sick plants, though with little or no success to date.
In more recent efforts to control the disease, two methods stand out. The first is the appearance of a genetically modified variety, called Rainbow, which is resistant to the Ringspot Virus, and was produced by a team of scientists headed by the University of Hawaii and Cornell University working together for more than a decade. They made it available in Hawaii in 1997, shortly after the risk of collapse in 1994. Although today more than 50 percent of the papaya plantations in Hawaii area are of this variety, the lack of consumer confidence in genetically modified products has prevented this variety from being completely accepted. It has lost important markets such as Japan, which before the introduction of this variety imported all of its papaya from Hawaii. Now the importation to Japan is minimal, and only of conventional papaya, since the Japanese market does not accept genetically modified papaya. Another problem with this approach are recent concerns about contamination by pollen of contiguous or nearby fields of conventional papaya. In spite of these problems, other genetically modified varieties have appeared. Some countries are developing their own genetically modified varieties. A significant drawback to many of these varieties is that they are only resistant to PRSV, while they remain vulnerable to other, less lethal but still dangerous, viruses such as the Apical Necrosis Virus, which requires constant applications of insecticides.
The second method of solving the problem, known as the Prophylactic Solution, involves a series of practices designed to slow down as much as possible the entrance in particular of the PRSV and in general of any virus into the orchard. This method was developed and is promoted by Cuban agricultural engineers specializing in the papaya. It requires cleansing the surrounding fields, eliminating possible host or carrier plants, perimeter fencing of the orchard with a border of live plants providing thick foliage, where the vectors clean their probe so that upon coming into contact with the papaya trees inside the orchard they do not infect them. It is necessary to cut the plants that show signs of PRSV and of other viruses and to maintain a constant application of insecticides to achieve the lowest possible levels of vector insects. Among the problems of this method are the difficulty in working in coordination with neighboring farmers, maintaining the living fences and accomplishing the necessary negative selection (elimination of infected plants). Although this would seem to be a favorable solution, it is difficult to impede the viruses, and in doing so correctly, to also have a normal harvest.
By way of background, fruits in nature typically develop from the ovaries of the flowers. The flowers may have feminine structures or masculine structures or both, such that there are feminine flowers, masculine flowers and those that contain both structures, which are called hermaphroditic flowers. The process of fruit formation starts with pollination, which is the transference of pollen from a masculine flower to the stigma of a feminine flower. The feminine flower must be prepared for fertilization through fecundation of the ovary, which must have one or two ova which through maturation will produce true seeds.
Pollen may be carried from one flower to the stigma of another in a variety of ways, e.g., by insects or carried by the wind. Many species of plants require pollen from different plants and this process is known as cross pollination, but some plants that have feminine or masculine or both structures in the same flower can fertilize themselves with their own pollen. The presence of feminine and masculine flowers or of flowers of both sexes does not necessarily mean a plant will self-pollinate since the ovary and the pollen may be viable at different times.
Only a few types of flowers in nature do not require pollination and fertilization to produce fruits. Such fruits are called parthenocarpic, and they do not contain seeds. In botany and horticulture, parthenocarpia means virginal fruit, and it is produced naturally in some plants and may also be induced naturally or artificially in other plants to produce fruits without fertilization of the ova in the flowers. Fruits created in this way, as stated above, do not contain seeds. Parthenocarpia occasionally occurs in nature as a mutation, but this mutation is usually considered a defect and the resulting fruits are frequently of poor quality and/or defective. Parthenocarpic fruits can be formed naturally (such as the navel orange) or artificially, by treating the flower with hormones (such as in some seedless tomatoes) or through other methods. Some parthenocarpic fruits have attained an extraordinary commercial value, such as in the case of the triploid watermelon, which does not contain seeds and is greatly preferred by consumers since it eliminates the inconvenience of removing the seeds when eating it.
Existing scientific literature regarding parthenocarpic papaya fruit in general is very poor, and regarding the maradol in particular is even less extensive. The few authors who write about it concur that parthenocarpic fruit is not recommended for commercial purposes due to the inconsistency in the way fructification occurs, which greatly diminishes the harvest. Other authors consider the almost universal smaller size and the supposed general bad quality to be a defect or error in fructification.
Finally, the Maradol is a papaya that was developed between 1938 and 1956 by the self-taught Cuban phytogeneticist, Adolfo Rodriguez Rivera, with the help of his wife, Maria Nodals. (The name Maradol was formed by combining the first letter of their names.) There are two types, the yellow and the red. The red type was introduced into México through research institutions in the 90's, and from there it became known in other parts of the world. For that reason technical and commercial reports say that it is from Mexico. Almost from its introduction it captured the attention of consumers for its organoleptic qualities, while marketers of papaya recognize its commercial qualities. At the present time, 80% of the area cultivated in papaya in Mexico is devoted to this variety, giving a strong boost to its production and placing it as the second producer in the world. This variety amply dominates the Mexican market, and here in the United States it captures ¾ parts of the market, which totals 300 million pounds annually. All of this Maradol fruit is imported from Mexico. It is very little known in Europe and in Asian countries. It is cultivated in Cuba and other countries of the Caribbean, Central and South America, and its expansion to the rest of the world is continuous. Among connoisseurs it is considered a fine papaya and a strong competitor in the world market.
Conventional processes for the production of Maradol Papayas typically involve cultivation under the open sky with a spacing between rows of plants of 2.5 to 3 meters and between 2 and 2.5 meters between plants within each row, with a density of 1330 to 2000 plants per hectare. Circular watering throughout the rows or drip irrigation is used, resulting in harvests of between 30 and 120 metric tons per hectare, depending on the level of technology used. The control of plagues and diseases is accomplished through abundant applications of chemical pesticides, and the nutrition is based on synthetic fertilizers. Weeding is manual, mechanical or with chemical herbicides. There is a constant risk of viral infections.